Many wireless digital communication systems construct data to be transmitted into a burst that contains all of the information required by a receiver to demodulate the data. This burst of information is commonly known as a packet, which includes three main components, namely a synchronization header, a physical layer (PHY) header, and a payload. The synchronization header comprises a preamble followed by a start frame delimiter (SFD). The preamble is typically a predetermined pattern, e.g. an alternating sequence of binary 1's and 0's, to enable the receiver to acquire bit timing and frequency synchronization. The start frame delimiter is a unique sequence of bits that signals the end of the synchronization header and establishes a timing epoch that indicates the start of the PHY header. The PHY header contains information that enables the receiver to demodulate the message data. The payload contains the data to be conveyed to the receiver.
The present invention is particularly directed to the reliable detection of the SFD in the synch header of an incoming packet. The ability to reliably detect the SFD can have a positive influence on the overall performance of the communication system. For instance, if the detection criteria are quite stringent, e.g. little or no errors are acceptable for a positive detection, packets may be lost due to the inability to obtain a perfect match in the presence of noise or channel impairments that are inherent to a wireless communication system. Conversely, if the detection threshold is lowered to accept some error, false detections result in the unnecessary consumption of processing resources directed to attempts to decode bits that are not part of an actual frame.
In general, detected bits in a received signal are matched against a known SFD pattern, for example by correlating the incoming bits with a copy of the SFD. A correlation peak that exceeds a predetermined threshold indicates the detection of the SFD in the incoming signal. Preferably, the results of the correlation should show a clear peak at the matching point, but remain as small as possible for all other points. Consequently, the sequence of bits should be selected to avoid near misses that could result in false detections.
In some networks, multiple SFD patterns may be employed to differentiate between different types of packets that are exchanged in a network. For example, some of the packets may be encoded with forward error correction (FEC), to enable errors in a received packet to be identified and corrected. Other packets may be transmitted in an uncoded format, with no error correction capabilities. To enable the receiver to determine which type of packet is being received, different SFD patterns can be respectively assigned to the two different types of packets. While the added flexibility provided by such an arrangement can increase the range of applications that are supported in the network, this enhanced flexibility should not occur at the cost of a reduction in overall system performance. For instance, the two SFDs should have the property that their cross correlation is very good, so that one SFD is not confused with the other during the detection process.
In networks that employ forward error correction to improve the reliability of the received data in the presence of channel impairments, such as fading and noise, different respective FEC mechanisms may be employed for the PHY header and the payload, to satisfy system requirements. In such an arrangement, the error resilience of both mechanisms should be approximately balanced, to ensure that neither the PHY header nor the payload becomes a clear limiter of overall system performance.
In the same way that the PHY header and payload error resilience should be balanced, so should the ability to detect the SFD. However, when powerful FEC codes are used on the PHY header and the payload, it is difficult to construct a robust method for SFD detection that matches such performance. One approach is to increase the length of the SFD to gain more reliability, but for theoretical reasons this results in diminishing returns and becomes impractical. Furthermore, system constraints may limit the SFD length.
Another approach to increasing the probability of detection is to lower the threshold at which a peak is declared to be a match with a desired SFD. However, this method results in false peaks being declared as matches, and undermines the anticipated gains of lowering the threshold.